Methods and apparatus for determining an etch endpoint in a plasma processing system

ABSTRACT

Methods and apparatus for ascertaining the end of an etch process while etching through a target layer on a substrate in a plasma processing system which employs an electrostatic chuck. The end of the etch process is ascertained by monitoring the electric potential of the substrate to detect a pattern indicative of the end of the etch process. By the way of example, changes to this potential may be observed by monitoring the current flowing to the pole of the electrostatic chuck. Upon ascertaining the pattern indicative of the end of the etch process, for example by monitoring the current signal, a control signal is produced to terminate the etch. If a bias compensation power supply is provided to keep the currents flowing to the poles of the electrostatic chuck substantially equal but opposite in sign throughout the etch, the compensation voltage output by the bias compensation power supply may be monitored for the aforementioned pattern indicative of the end of the etch process in order to terminate the etch.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the manufacture of semiconductordevices. More particularly, the present invention relates to improvedtechniques for ascertaining the end of an etch process for endpointingpurposes while etching through a selected layer on a substrate.

[0002] In the manufacture of semiconductor devices, such as integratedcircuits or flat panel displays, the substrate (e.g., the wafer or theglass panel) may be processed in a plasma processing chamber. Processingmay include the deposition of layers of materials on the substrate andthe selective etching of the deposited layer(s). To prepare a layer foretching, the substrate surface is typically masked with an appropriatephotoresist or hard mask. During etching, a plasma is formed from theappropriate etchant source gas to etch through regions unprotected bythe mask. The etching is terminated once it is determined that thetarget layer is etched through. This termination of the etch istypically referred to as the etch “endpoint.”

[0003] To determine when to terminate an etch, many techniques have beenemployed in the art. By way of example, the etch may be terminated uponthe expiration of a predefined period of time. The predefined period oftime may be empirically determined in advance by etching a few samplesubstrates prior to the production run. However, there is no allowancemade for substrate-to-substrate variations as there is no feedbackcontrol.

[0004] More commonly, the end of an etch process may be dynamicallyascertained by monitoring the optical emission of the plasma. When thetarget layer is etched through, the optical emission of the plasma maychange due to the reduced concentration of the etch byproducts, theincreased concentration of the etchants, the increased concentration ofthe byproducts formed by reaction with the material(s) of theunderlayer, and/or due to the change in the impedance of the plasmaitself.

[0005] It has been found, however, that the optical emission-basedtechnique has some disadvantages. By way of example, the use of someetchants and/or additive gases interferes with the optical emissionendpoint technique, giving rise to inaccurate readings. As a furtherexample, as the feature sizes decrease, the amount of film exposed tothe plasma through openings in the mask is also reduced. Accordingly,the amount of byproduct gases that is formed from reactions with theexposed film reduces, rendering signals that rely on plasma opticalemission less reliable.

[0006] It has been found that, as the target layer etch is completed andthe underlayer is exposed to the plasma, the self-induced bias of thesubstrate may change. By way of example, for the etch of a dielectrictarget layer, the self-induced bias of the substrate is observed tochange as a conductive underlayer is exposed to the plasma. As a furtherexample, for the etch of a conductive target layer, the self-inducedbias of the substrate is observed to change when a dielectric underlayeris exposed to the plasma. By monitoring the change in the self-inducedbias of the substrate, the end of the etch process may be ascertainedfor endpointing purposes.

[0007] To facilitate discussion, FIG. 1 illustrates a typicalendpointing arrangement wherein the self-induced bias on the wafer ismonitored to determine when the target layer is etched through for thepurpose of endpointing the etch. As shown in FIG. 1, a wafer 102 isshown disposed on an electrode 104, which is typically made of ametallic material. Electrode 104, which functions as a chuck in thisexample, is energized by an RF power source 106 through a capacitor 108.During etching, the self-induced bias on wafer 102 is detected at a node110 through a monitoring circuit 112. Monitoring circuit 112 include alow pass filter 114, which blocks the RF component of the signal andallows only the DC component to pass through. Since the self-inducedbias on the wafer tends to be in the hundreds of volts, the signal thatis passed through low pass filter 114 is typically stepped down througha voltage divider circuit to allow the monitoring electronics (not shownto simplify the discussion) to monitor the change in the self-inducedbias on wafer 102. This information pertaining to changes in theself-induced bias on the wafer allows the endpointing electronics todetermine when the etch should be terminated.

[0008] However, the sensitivity and accuracy of the monitoring techniquediscussed in FIG. 1 may degrade as the percentage of the target filmexposed to the plasma decreases and/or if the DC conductivity betweenthe plasma and the electrode is decreased (e.g., due to the presence ofa dielectric layer underlying the target layer to be etched).Furthermore, the monitoring technique of FIG. 1 is typically ineffectivewhen electrostatic chucks are employed. This is because electrostaticchucks typically employ a dielectric layer between the conductive chuckbody and the substrate. The presence of this dielectric layer interfereswith the current path between the plasma and the chuck, rendering itvery difficult to accurately determine the self-induced bias on thewafer at node 110. Furthermore, the relationship between the voltagedetected at node 110 and the self-induced bias on wafer 102 is notlinear. By way of example, the resistance of the electrostatic chuckdepends, in part, on the voltage existing on the chuck. Accordingly evenif a signal can be detected at node 110, it is difficult to correlatethe signal detected with the self-induced bias on the substrate forendpointing purposes.

[0009] In view of the foregoing, there are desired improved techniquesfor detecting the end of a plasma etch process for endpointing purposes.

SUMMARY OF THE INVENTION

[0010] The invention relates to methods and apparatus for ascertainingthe end of an etch process while etching through a target layer on asubstrate in a plasma processing system. This invention exploits thechange in the electric potential of the substrate which, for manydifferent etch applications, corresponds to the end of the etch process.In one embodiment, the endpointing arrangement includes a currentmonitoring circuit configured to monitor the current flowing to a poleof the electrostatic chuck to detect a pattern indicative of the end ofthe etch process. Upon ascertaining the pattern indicative of the end ofthe etch process in the current signal, a control signal is produced toterminate the etch.

[0011] In another embodiment, the chuck represents a bipolarelectrostatic chuck and currents flowing to both poles of theelectrostatic chucks are monitored for the aforementioned patternindicative of the end of the etch process in order to terminate theetch. In yet another embodiment, the differential of the currentssupplied to the poles of the electrostatic chuck is monitored for theaforementioned pattern indicative of the end of the etch process inorder to terminate the etch.

[0012] In yet another embodiment, the electrostatic power supplyincludes a bias compensation power supply, which monitors currentssupplied to the electrostatic chuck poles and outputs a compensationvoltage responsive thereto. The compensation voltage is then input intothe chuck power supply in order to keep the currents supplied to thepoles substantially equal but opposite in sign throughout the etch. Inthis embodiment, the compensation voltage is monitored for theaforementioned pattern indicative of the end of the etch process inorder to terminate the etch.

[0013] These and other advantages of the present invention will becomeapparent upon reading the following detailed descriptions and studyingthe various drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention is illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements and in which:

[0015]FIG. 1 illustrates a typical endpointing arrangement wherein theself-induced bias on the wafer is monitored to determine when the targetlayer is etched through for the purpose of endpointing the etch.

[0016]FIG. 2 is a simplified illustration of a compensation arrangementfor keeping the currents supplied to the chuck poles substantially equalin magnitude but opposite in sign as the etch progresses.

[0017]FIG. 3 illustrates a typical compensation voltage as the etchprogresses through the target layer.

[0018]FIG. 4 illustrates, in accordance with one embodiment of thepresent invention, a simplified arrangement for monitoring thecompensation voltage for the purpose of endpointing the etch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The present invention will now be described in detail withreference to a few preferred embodiments thereof as illustrated in theaccompanying drawings. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one skilled inthe art, that the present invention may be practiced without some or allof these specific details. In other instances, well known process stepsand/or structures have not been described in detail in order to avoidunnecessarily obscuring the present invention.

[0020] It is appreciated by the inventors herein that as the etchprogresses through a target layer, and particularly as the target layeris etched through to the underlayer, the electric potential of thesubstrate changes. The change in the substrate potential is particularlypronounced at the end of the etch. While not wishing to be bound bytheory, it is believed that, as the target layer is etched through, thecapacitive and resistive coupling between the substrate and the plasmachanges. As one possible explanation, the self-induced bias on thesubstrate may change due to the increased current leakage between theplasma and the substrate as the etch features (such as vias or trenches)are etched down to a stop layer. It is also possible that the propertiesof the plasma itself are changed as the target layer is etched through.This change brings about a change in the plasma impedance, which in turnchanges the self-induced bias on the substrate.

[0021] When an electrostatic chuck is employed in the plasma processingsystem, direct measurement of the substrate electric potential isdifficult, because the dielectric layer of the ESC introduces a largeresistance between the substrate and the electrical measurementcircuitry. The present invention overcomes these difficulties.

[0022] It is appreciated by the inventors herein that changes in thesubstrate electric potential cause variations in the current flowingfrom the ESC power supply to the poles of the electrostatic chuck. Inone of the embodiments of the present invention, the currents flowing tothe poles of the electrostatic chuck are monitored. In this manner, thechange of substrate potential associated with the end of the etchprocess may be ascertained, and the information derived therefrom may beemployed to endpoint the etch.

[0023] More preferably, some electrostatic chuck power supplies employ acompensation circuit to keep the currents flowing to the poles of theelectrostatic chucks substantially equal in magnitude but opposite insign. Compensation circuits are employed since if electrostatic forcesbetween the chuck poles and the overlying substrate regions vary duringan etch, inconsistent chucking, inconsistent heat transfer, andundesirable etch results may occur. In some systems, however, thecompensation circuit may be employed to keep the currents flowing to thepoles of the electrostatic chuck substantially constant (i.e.,relatively unchanging even if they are unequal throughout the etch).

[0024] In general, the compensation circuit typically monitors thecurrents flowing to the poles of the electrostatic chuck and provides acontrol signal to a variable bias compensation power supply. When thecurrents flowing to the poles of the electrostatic chuck poles change,the changing control signal varies the voltage output by a biascompensation power supply. The voltage output by the bias compensationpower supply, referred to herein as the compensation voltage, is thenemployed to offset the voltages supplied to the chuck poles in order tokeep the currents flowing to the electrostatic chuck poles substantiallyequal in magnitude but opposite in sign (or substantially constant inother systems as mentioned earlier).

[0025] It is discovered by the inventors that the compensation voltagechanges as the etch progresses and typically changes dramatically as thetarget layer is cleared, i.e., etched through. In accordance with oneembodiment of the present invention, information regarding end of theetch process may be obtained by monitoring the compensation voltage inorder to endpoint the etch.

[0026] To facilitate discussion, FIG. 2 is a simplified illustration ofa compensation arrangement for keeping the currents supplied to thechuck poles substantially equal but opposite in sign as the etchprogresses. It should be kept in mind, however, that while thecompensation arrangement of the exemplary embodiment functions to keepthe currents supplied to the chuck poles substantially equal butopposite in sign, the concepts disclosed herein also apply equally tocompensation arrangements that keep the currents flowing to the polessubstantially unchanging (i.e., relatively unchanging even if they areunequal throughout the etch). The adaptation of the exemplaryarrangement to work with such a compensation circuit is well within theskills of one of ordinary skills in the art given this disclosure.

[0027] With reference to FIG. 2, the object to be processed 200, e.g. awafer or glass panel, includes the target layer to be etched, and isrepresented in a simplified manner by a photoresist mask layer 202, atarget layer 204, underlayer film or films 206, and the substrate 207.Target layer 204 may represent any layer to be etched through. In oneexample, target layer 204 represents a silicon dioxide-containing layersuch as a doped CVD (chemical vapor deposition) or PECVD(plasma-enhanced chemical vapor deposition) glass layer. In anotherexample, target layer 204 may represent a low dielectric constant (low-kdielectric) layer. In yet another example, target layer 204 represents ametal layer or polysilicon (doped or undoped) to be etched. Underlayerfilm or films 206 may include any and all layers and/or structures thatunderlie target layer 204. Underlayer film or films 206 may include, forexample, one or more conductive (metallic or doped polysilicon) layersand/or one or more dielectric layers. By way of example, an etch stoplayer may be disposed immediately below target layer 204 and may beformed of, for example, silicon nitride, titanium silicide, or titaniumnitride material. Substrate 207 represents the supporting material ofthe object to be etched, for example, a wafer or glass panel. For thesake of discussion in the present example, substrate 207 does notinclude the layers and/or device structures which may be present on itssurface, which are instead represented by the aforementioned layers 202,204, and 206. In some cases, the underlayer film or films 206 may beabsent, and the target layer 204 is disposed directly on the substrate207.

[0028] In the example of FIG. 2 a Johnsen-Rahbek chuck is employedalthough the invention is believed to work with any type ofelectrostatic chuck such as monopole ESC chucks, multipole ESC chucks ofany configuration, or the like. The construction of a Johnsen-Rahbekchuck is well known in the art and will not be discussed in detail herefor brevity's sake. Further, although the chuck poles are of aconcentric configuration in the example of FIG. 2, the poles of theelectrostatic chuck may assume any configuration and/or geometry (e.g.,inter-digitated). For the concentric Johnsen Rahbek chuck of the exampleof FIG. 2, an outer pole 208 and an inner pole 210 are embedded in aslightly conductive layer 212, which may be formed of, for example, aceramic material that is lightly doped for conductivity. An RF electrode214, which is disposed below slightly conductive layer 212, is typicallyformed of a metallic material and is coupled to an RF power supply 216through a capacitor 218. To facilitate chucking, the poles of chuck 220are coupled to an electrostatic power supply 222.

[0029] Electrostatic chuck power supply 222 includes a main power supply224, which supplies the DC chucking voltages to the poles of chuck 220.Low pass filters to 230 and 232 are interposed between poles 208 and 210and electrostatic chuck power supply 222 to couple main power supply 224to poles 208 and 210 of chuck 220 and to isolate RF power 216 from powersupply 222. Current monitoring circuits 234 and 236 are coupled inseries with the current paths between the poles of the electrostaticchuck and ESC power supply 222 to monitor the currents in these legs.

[0030] Each of current monitor circuits 234 and 236 may be implementedby a simple resistive arrangement, and the potential difference acrosseach may be ascertained to determine the current flowing to each ofpoles 208 and 210. The outputs of current monitor circuits 234 and 236are input into a comparator circuit 238, which may represent, forexample, a differential amplifier circuit. Comparator circuit 238outputs a control signal 240 for controlling a variable biascompensation power supply 242. Bias compensation power supply 242changes its output responsive to control signal 240. The output of biascompensation power supply 242 is employed to bias main power supply 224to keep the currents flowing to poles 208 and 210 substantially equal inmagnitude and opposite in sign. The arrangement of FIG. 2, including thebias compensation arrangement in electrostatic chuck power supply 222,is well known in the art.

[0031] As target layer 204 is etched through, the compensation voltageat node 250 changes as the compensation circuit attempts to keep thecurrents flowing to poles 208 and 210 substantially equal. It isappreciated by the inventors herein that the information contained inthe compensation voltage, which is found either in control signal 240 orat node 250 at the output of bias compensation power supply 242,includes information pertaining the progress of the etch andparticularly pertaining when the end of the etch occurs. This isbecause, as explained earlier, the electric potential of the substrate207 changes as the etch progresses, and causes the currents flowing toeach of the poles 208 and 210 to change. These changes are detected bycurrent monitor circuits 234 and 236 to produce a control signal 240,which serves as the feedback signal to bias compensation power supply242, whose job it is to bias main power supply 224 to keep the currentsflowing to poles 208 and 210 substantially equal.

[0032]FIG. 3 illustrates a typical compensation voltage as the etchprogresses through the target layer. At point 302, the etch begins oncompensation voltage plot 300. As the etch progress, the compensationvoltage changes. Although the change is illustrated in FIG. 3 by anincreasing compensation voltage, the compensation voltage may change inother ways, such as decreasing, as the etch progresses in othersubstrates. As the etch clears the target layer, a significant change inthe compensation voltage is typically observed. Although the end of theetch is evidenced by a steep upward slope in the vicinity of region 304in FIG. 3, the end of the etch may also be evidenced (in other etchprocesses) by a sharp downward slope, a spike or a sudden dip in thesignal. Irrespective of the exact shape of the compensation voltage plotat the time the etch ends, the end of the etch is typically evidenced bya clearly discernible change in the compensation voltage. The specificcharacteristic shape of the compensation voltage plot at the time theetch ends may be ascertained by performing sample etches on samplewafers. Thereafter, the monitoring circuitry may be instructed to lookfor the ascertained characteristic shape in the compensation plot thatsignals the end of the etch for endpointing purposes.

[0033]FIG. 4 illustrates, in accordance with one embodiment of thepresent invention, a simplified arrangement for monitoring thecompensation voltage for the purpose of endpointing the etch. In FIG. 4,the voltage at node 250 is input into endpoint monitoring circuitry 402,which outputs an endpoint signal 404 when the characteristic changeindicative of the end of the etch process is ascertained. Monitoringcircuitry 402 may represent, for example, programmable digital circuitrythat has been programmed to analyze the input compensation voltagesignal and to output a control signal 404 for endpointing the etchprocess. In one example, monitoring circuitry 402 represents a generalpurpose digital computer (e.g., a microcomputer) or a digital signalprocessor that has been programmed to analyze the digitized compensationvoltage signal for changes indicative of the end of the etch process.

[0034] In accordance with another embodiment of the present invention,it is also possible to monitor control signal 240 itself for changescharacteristic of the end of the etch for endpointing purposes. Inaccordance with yet another embodiment of the present invention, thecurrents through the legs themselves may be monitored (by, for example,monitoring the outputs of current monitor circuits 234 and 236) forchanges in the current(s) that are indicative of the end of the etchprocess. This latter embodiment is particularly useful for chucks whichdo not employ compensation circuitry.

[0035] In accordance with another embodiment of the present invention,the difference in currents through the pole legs may be monitoredindirectly by the current monitoring circuit 248, even in the absence ofpower supply 242.

[0036] As can be appreciated from the foregoing, many embodiments of theinvention take advantage of existing signals in the electrostatic chuckpower supply for the purpose of ascertaining when the end of the etchoccurs in order to terminate the etch. In an indirect manner, changes inthe currents supplied to the poles of the electrostatic chuck areemployed to ascertain the etch progress for endpointing purposes. Unlikeprior art techniques, the endpointing technique of the present inventiondoes not require directly monitoring the self-induced bias of thesubstrate through the electrode (as was done in the case of FIG. 1).Accordingly, the technique works even with electrostatic chucks, whichhas a nonconductive dielectric layer disposed between the wafer and thebody of the chuck.

[0037] In fact, the accurate determination of when the etch ends ispossible even if there is a nonconductive layer disposed between thechuck's metallic body and the target layer. The presence of thenonconductive dielectric layer, either as part of the electrostaticchuck or within the substrate, would presumably have caused the priorart endpointing circuitry of FIG. 1 to fail to accurately provide anendpoint signal since the prior art technique depends on the directmeasurement of the self-induced bias on the substrate through theelectrode for endpointing purposes. Additionally, one of ordinary skillsin the art would have assumed that the presence of a dielectric layer onthe surface of the electrode and/or under the target layer would blockthe electrical path, rendering the direct monitoring of the self-inducedbias on the substrate impossible and/or very difficult. Since thepresent invention does not rely on direct contact between the substrateand the electrode, the presence of a such a dielectric layer does notprevent the ascertaining of the end of the etch in the presentinvention.

[0038] It is also observed that the inventive endpointing technique ishighly sensitive and is capable of accurately providing endpointinginformation even when etching substrates having a small fraction (orpercentage) of the target layer exposed to the etching plasma. Thesensitivity appears to increase if a conductive layer, e.g., aconductive metal or doped polysilicon interconnect layer, is disposedbelow the target layer to be etched. As alluded to earlier, thesensitivity of the present technique is such that the end of the etchprocess may be ascertained even if there is a dielectric layer disposedunder the target layer. Furthermore, since endpointing does not dependon monitoring the optical emission of the plasma, the inventivetechnique also works irrespective of the etchant and/or additive gasemployed.

[0039] While this invention has been described in terms of severalpreferred embodiments, there are alterations, permutations, andequivalents which fall within the scope of this invention. In general,it is proposed that the endpoint data can be derived from the changes inthe substrate potential, which can in turn be obtained by looking atvarious signals at various points in the system. Thus, although theendpoint data can be ascertained by monitoring the changes in thecurrent(s) flowing to the pole(s) of the ESC chuck (which reflect thechanges in the substrate potential), there are other ways of obtainingthis substrate potential-based endpoint data when an ESC chuck isinvolved. By way of example, a probe which contacts the backside of thesubstrate or some appropriate place on the substrate may be employed tomeasure the substrate potential directly throughout the etch, and theprobe signal may be analyzed for changes indicative of the etchtermination for endpointing purposes.

[0040] As another example, the leakage flow rate of coolant gas from theedges of the ESC chuck may be monitored during the etch, as an indirectmeasure of the substrate electric potential. This flow rate is dependentupon the clamping force of the ESC, which is, in turn, dependent uponthe potential difference(s) between the ESC and the substrate. As theetch proceeds, detectable changes in the flow rate may arise due tochanges in the substrate potential. In one embodiment, the leakage flowrate may be monitored in conjunction with or as part of a pressurecontrol arrangement which supplies the coolant gas to the interfacebetween the substrate and the ESC. The flow rate signal may be analyzedfor changes indicative of the etch termination, for endpointingpurposes.

[0041] In fact, given this disclosure, one of ordinary skills in the artwill readily recognize that changes in the substrate potential impactother signals at various points in the plasma processing system. Withthe knowledge imparted by this disclosure, the identification of thepossible signals and locations in a specific plasma processing systemthat may be monitored to ascertain the changes in the substratepotential is well within the skills of one familiar with plasmaprocessing equipment. It should also be noted that there are manyalternative ways of implementing the methods and apparatuses of thepresent invention. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutations,and equivalents as fall within the true spirit and scope of the presentinvention.

What is claimed is:
 1. An endpointing arrangement for ascertaining anend of an etch process while etching through a target layer on asubstrate in a plasma processing system, comprising: an electrostaticchuck having a first pole and a second pole; a first DC power supplycoupled to said first pole and said second pole for supplying chuckingvoltages to said first pole and said second pole; a first currentmonitoring circuit coupled between said first pole and said first DCpower supply for monitoring a first current supplied to said first pole,said first current monitoring circuit outputting a first signalindicative of said first current; a second current monitoring circuitcoupled between said second pole and said first DC power supply formonitoring a second current supplied to said second pole, said secondcurrent monitoring circuit outputting a second signal indicative of saidsecond current; a differential amplifier arrangement coupled to saidfirst current monitoring circuit and said second current monitoringcircuit, said differential amplifier arrangement receives said firstsignal and said second signal as input and outputs a control signal; avariable DC power supply coupled to said differential amplifierarrangement for receiving said control signal, said variable DC powersupply being configured to output a compensation voltage for biasingsaid first DC power supply responsive to said control signal; and anendpoint monitoring circuit coupled to said variable DC power supply,said endpoint monitoring circuit receives as an input said compensationvoltage and includes circuitry for analyzing said compensation voltagefor a pattern characteristic of said end of said etch process, saidendpoint monitoring circuit further including circuitry for outputtingan endpoint signal indicative of said end of said etch process uponascertaining said pattern in said compensation voltage.
 2. Theendpointing arrangement of claim 1 wherein said compensation voltagebiases said first DC power supply to maintain said first current andsaid second current substantially constant during said etch process. 3.A method for ascertaining an end of an etch process while etchingthrough a target layer on a substrate in a plasma processing system,said plasma processing system including an electrostatic chuck having afirst pole, a first DC power supply coupled to said first pole forsupplying a chucking voltage to said first pole, a first currentmonitoring circuit coupled between said first pole and said first DCpower supply for monitoring a first current supplied to said first pole,said first current monitoring circuit outputting a first signalindicative of said first current, and a variable DC power supplyconfigured to output a compensation voltage for biasing said first DCpower supply responsive to said first signal, thereby causing saidchucking voltage to vary responsive to said compensation voltage, saidmethod comprising: coupling an endpoint monitoring circuit to saidvariable DC power supply, said endpoint monitoring circuit having anendpoint monitoring input and an endpoint monitoring output; receivingat said endpoint monitoring input said compensation voltage; analyzing,using said endpoint monitoring circuit, said compensation voltage for apattern characteristic of said end of said etch process; and outputtingat said endpoint monitoring output an endpoint signal indicative of saidend of said etch process upon ascertaining said pattern in saidcompensation voltage.
 4. The method of claim 3 wherein saidelectrostatic chuck includes a second pole coupled to said first DCpower supply, said plasma processing system includes a second currentmonitoring circuit coupled between said second pole and said first DCpower supply for monitoring a second current supplied to said secondpole, wherein said compensation voltage output by said variable DC powersupply is responsive to both said first signal and a second signaloutput by said second current monitoring circuit, said second signalbeing indicative of said second current.
 5. The method of claim 4wherein said plasma processing system includes a differential amplifierarrangement coupled to said first current monitoring circuit and saidsecond current monitoring circuit, said differential amplifierarrangement receives said first signal and said second signal as inputand outputs a control signal to said variable DC supply to cause saidcompensation voltage output by said variable DC power supply to varyresponsive to both said first signal and said second signal.
 6. Themethod of claim 4 wherein said first signal and said second signal isemployed by said variable DC power supply to maintain said first currentand said second current substantially constant during said etch process.7. The method of claim 4 wherein said endpoint monitoring circuitincludes a general purpose microcomputer.
 8. The method of claim 4wherein said electrostatic chuck represents a Johnsen-Rahbek chuck. 9.The method of claim 4 wherein said substrate includes a conductive layerunderlying said target layer.
 10. The method of claim 4 wherein saidsubstrate includes a dielectric layer underlying said target layer. 11.The method of claim 4 wherein said target layer represents a silicondioxide-containing layer, said substrate further includes a dielectriclayer underlying said target layer.
 12. The method of claim 4 whereinsaid target layer represents a low dielectric constant layer, saidsubstrate further includes a dielectric layer underlying said targetlayer.
 13. An endpointing arrangement for ascertaining an end of an etchprocess while etching through a target layer on a substrate in a plasmaprocessing system, comprising: an electrostatic chuck having a firstpole; a first DC power supply coupled to said first pole for supplying achucking voltage to said first pole; a first current monitoring circuitcoupled between said first pole and said first DC power supply formonitoring a first current supplied to said first pole, said firstcurrent monitoring circuit outputting a first signal indicative of saidfirst current; a variable DC power supply configured to output acompensation voltage for biasing said first DC power supply responsiveto said first signal, thereby causing said chucking voltage to varyresponsive to said compensation voltage; an endpoint monitoring circuitcoupled to said variable DC power supply, said endpoint monitoringcircuit receives as an input said compensation voltage and includescircuitry for analyzing said compensation voltage for a patterncharacteristic of said end of said etch process, said endpointmonitoring circuit further including circuitry for outputting anendpoint signal indicative of said end of said etch process uponascertaining said pattern in said compensation voltage.
 14. Theendpointing arrangement of claim 13 wherein said electrostatic chuckincludes a second pole coupled to said first DC power supply, saidendpointing arrangement further comprises: a second current monitoringcircuit coupled between said second pole and said first DC power supplyfor monitoring a second current supplied to said second pole, saidsecond current monitoring circuit outputting a second signal indicativeof said second current, wherein said compensation voltage output by saidvariable DC power supply is responsive to both said first signal andsaid second signal.
 15. The endpointing arrangement of claim 14 whereinsaid compensation voltage output by said variable DC power supply isconfigured to keep said first current and said second currentsubstantially equal but opposite in sign.
 16. The endpointingarrangement of claim 14 wherein said compensation voltage output by saidvariable DC power supply is configured to keep said first current andsaid second current substantially constant throughout said etchingirrespective whether magnitudes of said first current and said secondcurrent are equal.
 17. The endpointing arrangement of claim 14 furthercomprising a differential amplifier arrangement coupled to said firstcurrent monitoring circuit and said second current monitoring circuit,said differential amplifier arrangement receives said first signal andsaid second signal as input and outputs a control signal to saidvariable DC supply to cause said compensation voltage output by saidvariable DC power supply to vary responsive to both said first signaland said second signal.
 18. The endpointing arrangement of claim 14wherein said endpoint monitoring circuit includes a general purposemicrocomputer.
 19. The endpointing arrangement of claim 14 wherein saidelectrostatic chuck represents a Johnsen-Rahbek chuck.
 20. Theendpointing arrangement of claim 14 wherein said substrate includes aconductive layer underlying said target layer.
 21. The endpointingarrangement of claim 14 wherein said substrate includes a dielectriclayer underlying said target layer.
 22. The endpointing arrangement ofclaim 14 wherein said target layer represents a silicondioxide-containing layer, said substrate further includes a dielectriclayer underlying said target layer.
 23. An endpointing arrangement forascertaining an end of an etch process while etching through a targetlayer on a substrate in a plasma processing system, comprising: anelectrostatic chuck having a first pole; a first DC power supply coupledto said first pole for supplying a chucking voltage to said first pole;a first current monitoring circuit coupled between said first pole andsaid first DC power supply for monitoring a first current supplied tosaid first pole, said first current monitoring circuit outputting afirst signal indicative of said first current; an endpoint monitoringcircuit coupled to said first current monitoring circuit, said endpointmonitoring circuit receives as a first input said first signal andincludes circuitry for analyzing said first signal for a patterncharacteristic of said end of said etch process, said endpointmonitoring circuit further including circuitry for outputting anendpoint signal indicative of said end of said etch process uponascertaining said pattern in said first signal.
 24. The endpointingarrangement of claim 23 wherein said electrostatic chuck includes asecond pole coupled to said first DC power supply, said endpointingarrangement further comprises: a second current monitoring circuitcoupled between said second pole and said first DC power supply formonitoring a second current supplied to said second pole, said secondcurrent monitoring circuit outputting a second signal indicative of saidsecond current, wherein said endpoint monitoring circuit receives as asecond input said second signal, said endpoint signal being responsiveto both said first signal and said second signal.
 25. The endpointingarrangement of claim 23 wherein said electrostatic chuck represents amonopolar electrostatic chuck.
 26. An endpointing arrangement forascertaining an end of an etch process while etching through a targetlayer on a substrate in a plasma processing system, comprising: anelectrostatic chuck having a first pole and a second pole; a first DCpower supply coupled to said first pole and said second pole forsupplying chucking voltages to said first pole and said second pole; afirst current monitoring circuit coupled between said first pole andsaid first DC power supply for monitoring a first current supplied tosaid first pole, said first current monitoring circuit outputting afirst signal indicative of said first current; a second currentmonitoring circuit coupled between said second pole and said first DCpower supply for monitoring a second current supplied to said secondpole, said second current monitoring circuit outputting a second signalindicative of said second current; a differential amplifier arrangementcoupled to said first current monitoring circuit and said second currentmonitoring circuit, said differential amplifier arrangement receivessaid first signal and said second signal as inputs and outputs adifferential output signal responsive to both said first signal and saidsecond signal; an endpoint monitoring circuit coupled to saiddifferential amplifier arrangement, said endpoint monitoring circuitreceives as an input said differential output signal and includescircuitry for analyzing said differential output signal for a patterncharacteristic of said end of said etch process, said endpointmonitoring circuit further including circuitry for outputting anendpoint signal indicative of said end of said etch process uponascertaining said pattern in said differential output signal.
 27. Theendpointing arrangement of claim 26 wherein said electrostatic chuckrepresents a Johnsen-Rahbek chuck.
 28. The endpointing arrangement ofclaim 26 wherein said substrate includes a conductive layer underlyingsaid target layer.
 29. The endpointing arrangement of claim 26 whereinsaid substrate includes a dielectric layer underlying said target layer.30. An endpointing arrangement for ascertaining an end of an etchprocess while etching through a target layer on a substrate in a plasmaprocessing system, comprising: an electrostatic chuck having a firstpole and a second pole; a first DC power supply coupled to said firstpole and said second pole for supplying chucking voltages to said firstpole and said second pole; a first current path coupled in parallel withoutputs of said first DC power supply; a first current monitoringcircuit coupled to a node in said first current path for monitoring afirst current flowing to said node, said first current monitoringcircuit outputting a first signal indicative of said first current; andan endpoint monitoring circuit coupled to said first current monitoringcircuit, said endpoint monitoring circuit receives as an input saidfirst signal and includes circuitry for analyzing said first signal fora pattern characteristic of said end of said etch process, said endpointmonitoring circuit further including circuitry for outputting anendpoint signal indicative of said end of said etch process uponascertaining said pattern in said first signal.
 31. The endpointingarrangement of claim 30 wherein said first current monitoring circuit iscoupled in series between said node and ground.
 32. The endpointingarrangement of claim 30 further comprising a variable DC power supply,said first current monitoring circuit is coupled in series between saidvariable DC power supply and said node.
 33. An endpointing arrangementfor ascertaining an end of an etch process while etching through atarget layer on a substrate in a plasma processing system, comprising:an electrostatic chuck having a first pole; a first DC power supplycoupled to said first pole for supplying a chucking voltage to saidfirst pole; an endpoint monitoring circuit configured to receive asignal reflecting a potential of said substrate, said endpointmonitoring circuit including circuitry configured to analyze said signalreflecting said potential of said substrate for a pattern characteristicof said end of said etch process, said endpoint monitoring circuitfurther including circuitry for outputting an endpoint signal indicativeof said end of said etch process upon ascertaining said signalreflecting said potential of said substrate.
 34. The endpointingarrangement of claim 33 wherein said signal reflecting said potential ofsaid substrate is obtained by a probe in contact with said substrate.